1. Field of the Invention
The present invention generally relates to a lens actuator used for a device (hereinafter called an optical disk drive device) including a plurality of values for different multiplied speeds for optically recording and reading information to a recording medium.
2. Description of the Related Art
In a lens actuator used for an active control, it is required to employ a movement mechanism for maximally reducing a loss operation caused by a non-linear characteristic. Thus, a spring support mechanism has been widely applied as a support mechanism of the lens actuator used in the optical disk drive device that requires a high accurate control. A detailed description of a mechanism supporting a light pickup by four support springs (linier elastic member) is found in Japanese Laid-Open Patent Application No.9-190636 (Japanese Patent No.2856176), for example.
On the other hand, since it is required to mount a high NA objective lens due to an increase of a record density, a tilt of an optical axis caused by an irregular rotation in a translational motion of focusing and tracking needs to be suppressed. However, according to the mechanism described in Japanese Laid-Open Patent Application No.9-190636 (Japanese Patent No.2856176), a compliance (elastic coefficient) in both directions of the support spring needs to be lower in order to improve displacement sensibilities of a focusing direction and a tracking direction. In addition, since an outside dimension is limited by minimization and be lightweight and an arrangement and a size of the support spring are limited, there is a problem such that a torsional rigidity in a radial direction is inevitably degraded due to a principal of this mechanism.
The irregular rotational motion in the radial direction is caused during the translational motion, by an assembly error, by an irregular moment of a driving thrust when a focusing operation and a tracking operation are translationally shifted, or by a slight asymmetry caused by an assembly error of a support spring. The occurrence of the irregular rotational motion causes characteristic problems and also decreases a yield and increases costs of production.
The tilt of the optical axis caused by the irregular rotational motion has been discussed and recognized mainly as a problem for a DC (Direct Current) shift of the lens actuator along with tilts of a radial axis and a tangential axis. First, as described above, the irregular moment is caused on an active radius of a thrust in a direction where a DC shift amount perpendicularly crosses. Second, a frequency of a resonance (hereinafter called a roll resonance) could be set higher than that of a maximum spindle rotation in a specification of a conventional optical disk drive device.
However, since a higher speed has been required recently for the optical disk drive device, the frequency of the spindle rotation tends to be higher than that of the roll resonance of the conventional lens actuator in a reproducing operation and a recording operation. When both frequencies correspond to each other, such as a phenomenon described later can be caused.
A surface vibration of the optical disk and an eccentric factor of a spectrum tend to intensively show low frequency factors in accordance with a natural principal and a primary factor, that is, a larger spindle rotation frequency factor exists in a disk standard. In detail, the surface vibration and the eccentric factor, which include the spindle rotation frequency factor of the disk, become a compulsory feedback to the lens actuator focusing and tracking and then becomes active as exciting forces.
There is no problem only when these exciting forces are active as a logically pure translational thrust. However, when the translational thrust is occurred to each of the focusing operation and the tracking operation, it is not possible to prevent a state in which the DC shift occurs in the tracking direction and the focusing direction that are perpendicular each other. As a result, this DC shift amount functions as the active radius and the translational thrust causes an irregular thrust moment. Also, since the irregular thrust moment has the same frequency factor as the roll resonance frequency due to the above-described frequency correspondence, a rotation resonance is caused by the exciting force of a synchronized signal.
A tilt amount in the radial direction by AC (Alternating Current) caused by the rotation resonance exceeds far above a peak value of a tilt amount in the radial direction conventionally caused by the DC shift, and modulates an amplitude of a tracking error signal or a RF (Radio Frequency) signal and then prevents a stable performance of recording or reading. Thus, a servo error can occur.
Since these above problems are related to the resonance, it is not effective to use a means of offsetting irregular moments of the focusing operation and the tracking operation, which are used for an irregular inclination in a consideration of the DC shift only.
Consequently, a synchronous signal vibration in the roll resonance frequency is a vibration condition to maximize the sensitivity of tilting in the radial direction. When the surface vibration of the optical disk or the AC exciting force for an eccentric follow-up additionally occurs in an inevitable cumulative deviation in the focusing direction or the DC shift such as a carriage follow-up error or a like, the tilt amount in the radial direction is rapidly increased. As a result, the amplitude of the RF signal is modulated and then an error rate is increased or an occurrence possibility of the servo error is increased. Accordingly, it is greatly prevented to improve a speed. It is difficult to realize the requirement of the higher speed in the lens actuator.
However, the roll resonance frequency always exists somewhere when the lens actuator is used. Thus, it is not possible to avoid the above disadvantageous features caused by the roll resonance frequency. For the sake of convenience, the above description is predicated on the spring support structure, but the same disadvantageous features occur on other structures such an elastic hinge or bearing. Accordingly, it is desired to avoid the spindle rotation frequency and also set the roll resonance frequency. It is the most desirable to set the roll resonance frequency higher than the spindle rotation frequency for a maximum number of activated rotations.
However, in response to a requirement of the high speed operation above-described, a rotation speed, which is required to the optical disk drive within the operation time of the lens actuator used for the optical pickup, is not always conducted in an identical condition. In addition, since the different spindle rotation number is controlled based on each of the reading operation and the recording operation for a CD (Compact Disc) or a DVD (Digital Versatile Disk).
Moreover, according to a principal of a natural vibration, the roll resonance frequency is proportional to a square root of a torsional spring constant in the spring support structure and also is inversely proportional to a square root of a moment of intertia in a movable structure. According to this principal, for the roll resonance frequency to be sufficiently high, it is required to set the torsional spring constant to be greatly higher than the conventional torsional spring constant, it is required to set the moment of intertia to be extremely smaller than the conventional moment of intertia, or both determinations are required. Generally in the spring support structure, when the support spring is set so as to be a lower compliance in order to make the DC sensitivity higher, the torsional spring constant can not be avoided to be lower because the torsional spring constant is dependent on a state of the compliance. On the other hand, in another support structure that is not likely to be influenced by such the state of the compliance, it is required to provide a pair of thrust generating sources at each of both sides for symmetry thereof since the support structure occupies a center area of the movable configuration. Consequently, each configuration must be large enough to support an increase in the moment of inertia.
As describe above, it is difficult to realize a desired roll resonance frequency alone that is sufficiently higher while satisfying other advantageous features in any one of the above configurations.
Furthermore, when it is impossible for a certain reason to set so as to fully avoid the roll resonance frequency in the actuator used for the driver using a plurality of different frequencies, it is required to fully reduce the AC tilt sensitivity itself. Since the AC tilt described here is the radial tilt sensitivity in a roll resonance region, that is, the AC tilt is a sum with a radial DC tilt factor, and a roll resonance magnification factor, in order to reduce the AC tilt, it is necessary to reduce a radial DC tilt factor, it is necessary to reduce the factor other than the radial DC tilt factor, that is, the resonance magnification in roll resonance, or it is needed to reduce the both.
However, even if a tilt sensitivity in a low cycle can be lower, in the conventional actuator, as for this, the resonance magnification in active natural oscillating frequency is 10 (dB) or more than that, that is, an amplitude is about 3 or more times of a low cycle factor. Since a damper material temporarily optimized for the natural-oscillating frequency and used for the actuator degrades an attenuation ability in a higher frequency than the natural-oscillating frequency, that is, the roll frequency, it cannot be avoided that the AC tilt sensitivity in the low frequency becomes greater than 3 or more times.
With the conventional technology, in order to reduce the resonance magnification, the damper material and its form are selected and used. In the actual condition that the irregular resonance exists in frequency higher than active resonance, in order to suppress the irregular resonance magnification to be lower than a fixed level, it is necessary to suppress the active resonance magnification to be considerably lower.
Recently, since the irregular resonance approaches to disk rotation frequency and an inclination amount of the irregular resonance becomes greater, it is more important to suppress the roll resonance. In order to suppress the roll resonance, the damper material is filled so that the resonance magnification in the active natural oscillating frequency can be suppressed less than 6 (dB).
Generally, a high viscous damper material may be used, an area of the filling part of the damper material may be increased, or the both may be employed. However, in this case, the equivalent spring constant factor, which the damper material has, increases rapidly and then the active DC sensitivity is reduced. Or, since the phase is delayed in the low frequency region by over attenuation in an active mode, even in a case of spring support structure, an unfavorable hysteresis characteristic is generated.